Infectious diseases constitute an overwhelmingly high proportion of illnesses in clinical practice, more so in the developing countries. Their impact on the morbidly and mortality can be considerably minimized by introducing rational intervention strategies including vaccination and effective therapy. This in turn would depend upon the depth of our knowledge regarding the Etiological agent, pathogenesis of the disease and detailed epidemiology. The germ theory of the disease became firmly established only in the late 19th century. The science of infectious diseases has been progressing steadily since then. The practice of the principles of vaccination environmental control of the diseases, and importance of personal hygiene nutritional management and availability of potable water has resulted in control of several infectious diseases of the past. While not minimising the impact of these contributions in the control of infections diseases, the past. While not minimizing the impact of these contributions in the control of infections disease, the fact remains that the conventional methods of disease diagnosis and epidemiological tools are slow and time consuming. With the advent of molecular biology techniques much more effective tools are now available, the impact to which may be regarded as nothing short of a revolution. The advances have been very rapid and have altered the way we look at the infectious agent. The opportunities to study the microorganism are unlimited, so much so that never before in the history of medicine such optimism has existed about our abilities to quickly and effectively combat infectious diseases in a cost effective manner.